Remembering Gerald Wasserburg

Gerald J. Wasserburg, John D. MacArthur Professor of Geology and Geophysics, Emeritus, passed away on June 13, 2016. He was 89 years old.

Wasserburg's work established a time scale for the development of the early solar system including the end of the process of nucleosynthesis—the process by which atoms heavier than hydrogen formed—and the formation about 4.5 billion years ago of solid objects such as the earth and the moon, other planets, and certain meteorites. He also is acknowledged widely for his isotope studies of lunar materials collected by the Apollo missions and his involvement in U.S. space research programs. He and his colleagues also did important work on the dating of rocks, on the evolution of the earth through time, and on the modern oceans.

Wasserburg was born in New Brunswick, New Jersey, on March 25, 1927. He served in the U.S. Army from 1943 to 1946, then graduated from high school. He enrolled at Rutgers University under the GI bill, and later transferred to the University of Chicago, from which he earned his SB (1951), SM (1952), and PhD (1954) degrees.

Wasserburg joined Caltech faculty in 1955 as an assistant professor of geology. He became an associate professor in 1959, professor of geology and geophysics in 1963, MacArthur Professor in 1982, and he retired in 2001. He served as chair of the Division of Geological and Planetary Sciences from 1987 to 1989 and as executive officer for geochemistry over the same time period.

Not long after his arrival at Caltech, Wasserburg began the work that he is most famous for: leading the construction of the Lunatic I, a mass spectrometer for making high-precision measurements of lunar samples obtained by the Apollo missions.

The Lunatic I—now held in the collections of the National Museum of American History—"revolutionized the field of geochemistry," says Donald Burnett, professor of nuclear geochemistry, emeritus, and one of Wasserburg's longtime colleagues. "It totally changed the world because the precision with which you measure ratios of isotopes was an order of magnitude greater than before."

Radioactive isotopes decay at a known rate into other isotopes. For example, about 10 percent of potassium-40 decays produce argon-40, with a half-life of 1.25 billion years. That means that one-half of the atoms of potassium-40 in a given sample—within a rock specimen recovered from the moon, for example—will decay over a 1.25 billion-year time span and about 10 percent of those decays yield argon-40; over the next 1.25 billion years, half of the remaining potassium-40 atoms will decay, and so on. Such rates of decay, which vary from isotope to isotope, provide a "clock" that can be used to determine the age of the specimen, provided you have an accurate accounting of the isotopes in your sample.

The Lunatic I was built at Caltech's Central Engineering Services and installed in a new lab on the second floor of the Arms Laboratory at Caltech that Wasserburg named the "Lunatic Asylum." Mass spectrometers separate ionized atoms according to their mass, and the Lunatic I—developed by Wasserburg and his then-graduate student Dimitri Papanastassiou (BS '65, PhD '70), now a faculty associate in geochemistry—was the first such instrument that was fully digital, with computer-controlled magnetic field scanning and rapid switching and digital ion beam measurement, enabling it to measure reliably isotope ratios with a precision that was a factor of 30 better than earlier instruments. "Those were exciting times," Papanastassiou says. "We were doing things that nobody else could do."

Wasserburg was invited in 1967 to be part of the team of scientists that would handle the initial processing of all of the lunar rocks collected by the Apollo astronauts. Together with Bob Walker of Washington University, Jim Arnold of the University of California at San Diego, and Paul Gast of Columbia University, he was one of the "Four Horsemen"—senior scientists who advised NASA during the Apollo missions.

The "inmates" of the Asylum (as Wasserburg referred to them) were among several teams of researchers to receive lunar rocks from the Apollo 11 mission, which, in July 1969, was the first to land humans on the moon. Each team had been tasked with independently studying the rocks—including to determine their age—with the results to be presented at the first Lunar Science Conference, held in Houston in January 1970.

At the conference, Wasserburg and his colleagues announced that the samples they had analyzed with Lunatic I were between 3.5 and 3.7 billion years old, with a high degree of accuracy. "Afterward, you saw everyone running around trying to quickly revise their talks," Burnett recalls. "They had numbers for the age of the rocks too, but their margins of error were too big to be meaningful."

Wasserburg continued research on lunar samples through the end of the Apollo program. The crew of the Apollo 13 mission—which was forced to abort its lunar landing because of an onboard explosion—gave Wasserburg a photo bearing the inscription, "Sorry we couldn't bring back any rocks." It remained on his wall for decades.

When President Richard Nixon moved to cancel the final Apollo missions, the Four Horsemen organized a protest from the scientific community. The Apollo 16 and 17 missions were reinstated.

In later research, Wasserburg discovered the presence of the decay products of the short-lived radioactive isotope aluminum-26 in the oldest remnants of the solar system, suggesting that a nearby supernova contributed matter to the solar nebula shortly before it collapsed and accreted to form the sun and planets. Lunatic 1 also played a key role in discovering the first evidence of the "late heavy bombardment", a period, roughly 4 billion years ago, when the moon was pummeled by asteroids and comets. Ultimately, his advances in dating techniques contributed to the timeline for the evolution of the solar system that we know today.

Colleagues remember Wasserburg as a driven and competitive scientist who was always excited about his work. "Every day, he really believed that the work he'd done in the lab yesterday was the best science of the past 10 years," Burnett says. Wasserburg's painstaking attention to detail was well known. "He had very high standards for himself and everyone else, and was strongly supportive of young scientists at the Institute" says Geoffrey Blake (PhD '86), professor of cosmochemistry and planetary sciences and professor of chemistry. "Those are the kind of people who make Caltech special."

Wasserburg was the recipient of the Crafoord Prize in Geosciences in 1986 and numerous other honors, including the Arthur L. Day Medal from the Geological Society of America (1970), the NASA Distinguished Public Service Medal (1972 and 1978), the Wollaston Medal of the Geological Society of London (1985), the Gold Medal of the Royal Astronomical Society (1991), and the William Bowie Medal of the American Geophysical Union (2008). He was a member of the National Academy of Sciences, the American Philosophical Society, the American Academy of Arts and Science, and the Norwegian Academy of Science and Letters.

He is survived by Naomi Wasserburg, his wife of more than 60 years, and his sons Charles and Daniel Wasserburg.